Method of heat treating magnetic materials



Jan. 5, 1937.

W. M. BISHOP METHOD OF HEAT TREATNG MAGNETIC MATERIALS Filed Jlily 6, 1934 CONTROL APMIMTUS /NVENTOR W. :1g/jl' HUP Arron/v5 Patented Jan. 5, `1937 PATENT OFFICE METHOD or' HEAT TREATING MAGNETIC MATERIALS p Walter M. Bishop, Flushing, N. Y., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application yJuly 6, 1934, Serial No. 733,956

' 1 claim. (c1. 14s-13) This invention relates to a method of improving the magnetic properties of magnetic materlals and more particularly to a method of increasing the permeability of thin sheets or laminations of magnetic material as used for the cores of transformers, loading coils, and inductances as well as diaphragms for telephone receivers and other parts of magnetic circuits. It has been known for some time that thin sections or laminations of approximately ten thousandths of an inch or less in thickness behave diierently magnetically than thicker laminations of the same material. The most noticeable dierence is the lower permeability of the thin laminations. The

permeability appears to decrease as the thicknesses decrease.

'I'he object of this invention is to provide a method whereby the permeability of these thin laminations may be materially increased.

A further object of the invention is to remove the stresses and strains in the completely formed laminations and laminated core.

In brief, the method consists in clamping a stack or core of completely formed laminations between blocks of the same material as said laminations or a material having substantially the same coeillcient of expansion as said laminations and then heat treating the clamped stack of laminations.

In order that the invention may be more fully understood an embodiment of it will now bedescribed with reference to the attached drawing yof which:

Fig. 1 shows one method of practicing this inventlon on one specific type of core; and

Fig. 2 shows another arrangement for practicing the invention.

'Ihe term lamination as used in this specica= tion refers to any thin sheet, member, or section 40 of a magnetic circuit and includes cores-'or portions of cores for transformers, loading coils, inductances, relays, electromagnets, .and also thin sections or portions of magnetic circuits .such yas dlaphragms for telephone receivers.

Referring to Fig. 1, I0 representsa stack of laminations which are clamped between blocks 2 2. These laminations may be individually dusted or insulated with some refractory powder such as quartz dust to prevent them from sticking together. Any suitable number may be clamped ln a stack; as for example, the number required for a single core; some extra laminations in addition to-those necessary for a complete core, or enough laminations to form a number of cores 5p may be stacked and clamped between the blocks 2 2. If these blocks or washers 2 2 are not of suiilcient thickness, other blocks -or washers 4 4 may be provided to further increase the strength .of washers 2 2 so that they will not be deformed by the clamping action of bolt 5 and nut 6. 5 Blocks 2 2 may be of the same dimensions or area as the laminations or of any other suitable dimensions. In case washers 2 2 are of suilicient thickness and strength, it will not be necessary to provide the additional washers or blocks 4 10 to more evenlyldistribute the clamping forces to e the laminations I0. After the laminations have' been placed between blocks 2 2 which in turn are placed between blocks 4 4, bolt 5 is inserted and nut 6 tightened to firmly clamp laminations 15 I0 between the blocks 2 2 with a 'greater force than the weight of the laminations and clamp. The laminations are then heat treated by placing them in a heat treating furnace. After the ma- -terlal has been heated, itis usually allowed to cool 20 slowly.

A suitable furnace for heat treating clamped laminations is represented by 8 in Fig. 1. Top 9 is provided to completely enclose the laminations. In case it is desired to heat treat the material in. 25 a controlled atmosphere, such as a N2,H2,A, He, Ne, CO, CO2CH1 or other inert or hydrocarbon gases and vapors either alone or in combination atmosphere, top 9 may be sealed to furnace 8 at Il in any suitable manner. Pipe I2 is then con- 30 nected to a supply of gas or vapor to be used during heat treatment. The excess escapes at i3 where it may be burned if lt is combustible. The furnace may be heated in any manner, as for example, by heating wires I 4 which are connected 35 to a source of electrical power through switching andregulating mechanism I5. The material ls thus heated to a high temperature in a controlled atmosphere after which it is usually allowed to slowly cool in this atmosphere, but need not be so 40 cooled.

In casel the laminations to be treated are in the form of diaphragms, it may be desirable to employ a plurality of washers 2 2 of the same material as the laminations or diaphrag-ms. 45 These washers are distributed throughout the stack between the laminations to insure thatv each lamination or diaphragm will be. very at after the heat treatment.

This arrangement is illustrated in Fig. 2. At 5u the top of this stack a washer 2 is inserted between each ive laminations or sheets while at the bottom of the stack a washer ls placed between each lamination. It is to be understood, however, that as many or as fev,1 washers 2 are provided as are 55 needed. In case it is necessary to have extremely flat laminations or sheets, it may be desirable to provide a washer between each lamination as shown in the bottom of the stack of Fig. 2. In other cases one washer every ilvev or more laminations may be all that is required.

If the material of washers 2 is different or has a diierent coeillcient of expansion than that of laminations I0, frictional forces between the laminations and the washers will cause stresses and strains to be set up within laminatlons I0 during the cooling time due to the change in the relative dimensions of the laminations and washers. These strains will greatly reduce the permeabil- .ity of the laminations. By using blocks 2--2 of the same material or a material having the same thermal coeilicient of expansion as the laminations I0, these forces and thus these stresses are largely eliminated.

In order that the stresses and strains may be further eliminated, if necessary, ythe surfaces of blocks 2 2, such as 3, may vbe made smooth or polished so that small frictional forces, that may be due to differences in cooling rates of washers 2--2 and laminations I0, may be easily removed by slipping or sliding of the washers relative to laminations I. 'I'hus the stresses or strains will not be transferred to the laminations to cause a reduction of their permeability.. In some cases the other surface 1 of blocks 2-2 and even the adjacent surfaces of block 4 may be polished if desirable. 'Ihis readily enables relative motion to be made between the various components of the clamp to relieve the stresses and strains set up due to differences in the cooling rates and the dierent coefilcients of expansion of the various members of the clamp. In other cases it may be desirable to laminate washers 2--2 or employ more than one on-each side of the stack of core .laminations as shown at the ends ofthe stack in Fig. 2. In addition, it may be necessary to discard some of the lamlnations adjacent to the clampingsurfabes if the frictional forces between the clamping surfaces and adjacent laminations are still large enough to materially reduce the permeability of these laminations. These discarded laminations may be used repeatedly as the laminated washers to take up all the frictional forces tending to strain the core laminations during cooling. However, thediscarded laminations may be reheat treated'in the next lot and then utilized as magnetic sheets.

This method of heat treating thin lamina-,

tions for magnetic cores has the further advantage that the surfaces of these laminations are in contact with similar materials and thus do not readily become contaminated from the clamping blocks, refractories or other material including furnace gases. Neither does the material nor its constituents evaporate during the-heat treatment. The composition, therefore-willlremain substantially the same throughout this lamina- This manner of treating laminations has been found tp materially increase the permeability of thin laminations. For example, in the specific case of laminations three thousandths of an inch thick ofan alloy comprising 78.1,% nickel, 3.8% molybdenum and the remainder iron when heat treated in the normal manner have an initial permeability of less than 7000. However, when laminations of this same material only one thousandth of an inch thick are heat treated when clamped together as described above, they have an initial permeability of approximately 10.000 whichisveryclosetothepermeabilityofthe material in much thicker sections or laminations.

In another case laminations approximately two thousandths of an inch thick made of an alloy comprising approximately 45% nickel and the remainder chiefly iron were clamped together in 5 accordance with this invention and heated in a furnace which was maintained at 1000 C. for an hur and then allowed to slowly cool. A hydrogenous atmosphere was maintained in thefurnace during this time. The initial permeability of 1 these laminations after this treatment is about the same as for thicker laminations and is given in the following Table A.

Table A.

Specimen h 1 htly clam 'rightly damped 'Ihis manner of treating magnetic materials will also improve other magnetic properties in addition to the permeability. For example, the iron-nickel-cobalt alloys characterized by a substantially constant permeability at low magnetic fields are improved by a heat treatment in accordance with this invention. 'I'hese alloys usually react to various treatments in an opposite manner to the usual magnetic materials. The initial permeability no is increased by cold working, strains produced by reducing them to thin laminations. In addition, the permeability is not constant at low fields. The constancy is sometimes designated i which is denned as nos where ,lo is the dinitial permeability and n the permeability in the field in which the material is used or tested and Bm the maximum value of induction in the material in this ileld. 'Ihe following Table B lists these factors for laminations two thousandths of an inch thick made from an v alloy comprising approximately 37% nickel, 25% cobalt, 7 .5% molybdenum, and the balance chiefly iron which have been clamped together in accordance with this invention and heated in a hydrogenous atmosphere in a furnace which was maintained at 1000 C. for one hourand then allowed to slowly cool.

Table B A method of manufacturing a magnetic core composed of a considerable number of lamina.- tions of highly` strain sensitive magnetic material less than 0.003 inch in thickness which com- 4 prises stacking a. sumcient number of completely formed laminations together to tormf a. complete magnetic core, forcibly clampingsaid complete 

